8.4 Reproduction

Fertilization

There are two types of gametes involved in reproduction: the male gametes, called sperm, and female gametes, called ova. The male gametes are produced in the testes through a process called spermatogenesis, which begins at about 12 years of age. The female gametes are present at birth but are immature. They are stored in the ovaries until puberty, when one ovum ripens and is released about every 28 days in a process called oogenesis.^[1]

After the ovum is released from the ovary, it is drawn into the Fallopian tube. If sexual intercourse occurs, millions of sperm are ejaculated into the vagina, but only a few reach the ovum. Fertilization occurs when a sperm and an ovum combine, also referred to as conception. See Figure 8.14^[2] for an image of conception.

After a single sperm has entered the ovum, the wall of the ovum becomes hard and prevents other sperm from entering. The head of the sperm, containing the genetic information from the father, unites with the nucleus of the ovum that contains genetic information from the mother, and a new cell is formed. Because each of these reproductive cells is a haploid cell containing half of the genetic material needed to form a human being, their combination forms a diploid cell called a zygote.^[3]

During the first week after conception, the zygote divides and multiplies, going from a one-cell structure to two cells, then four cells, then eight cells, and so on. This process of cell division is called mitosis. The zygote continues to divide and travel through the Fallopian tube for about seven to ten days, at which point it arrives in the uterus. Once in the uterus, the zygote implants itself in the endometrium, the inner lining of the uterus, where the embryonic stage begins about three weeks after conception.^[4]

Human chorionic gonadotropin (HCG) is a hormone produced by cells that will eventually make up the placenta in the early stages of pregnancy. HCG can be detected by a blood pregnancy test at around 7-12 days after conception and by a urine pregnancy test about two weeks after conception. HCG increases quickly (almost doubling every three days) for the first eight to ten weeks of pregnancy.^[5]

Embryonic Stage

An embryo is an organism in its early stages of development, specifically from Day 16 through Week 8. See Figure 8.15^[7] for an image of a human embryo.

During the embryonic stage, blood vessels grow, forming the placenta. The placenta is a structure that connects the fetus to the uterus, providing oxygen and nutrients from the mother to the developing embryo via the umbilical cord. Basic structures of the embryo start to develop into areas that will become the head, chest, and abdomen. During the embryonic stage, the heart begins to beat, and organs form and begin to function.^[8]

The embryonic stage of fetal development begins with the completion of implantation and ends at the start of Week 9 of gestation. The weeks of gestation during pregnancy are calculated from the first day of the pregnant person’s last menstrual period rather than from the date of conception. In the final days of the pre-embryonic stage, the blastocyst develops into three germ layers called the ectoderm, mesoderm, and endoderm. The formation and development of the organs of the body, called organogenesis, starts at the beginning of the embryonic stage. The three germ layers develop into the organs, tissues, and structures of the body.^[9]

Although the nervous system starts forming first, the embryo’s cardiovascular system begins functioning first. The hemoglobin formed by the embryo has a higher affinity for oxygen than adult hemoglobin because the embryo is dependent on the parent for oxygen. The embryo also requires more oxygen to support its rapid growth. The nervous system works within the first eight weeks of gestation but is not fully developed at birth. This is evident by a newborn’s lack of neuromuscular coordination, developed senses, or speech. The brain is not fully developed until five years of age. In most cases, the gonads differentiate into male or female by eight weeks. The testes stay in the inguinal canal, fully descending between 34 and 36 weeks of gestation.^[10]

Exposure to hazardous agents called teratogens during organogenesis increases risk of malformations in all systems of the embryo’s body. For example, during organogenesis, the trachea and esophagus are one tube, and the separation into esophagus and trachea is not complete until five weeks of gestation. If the pregnant person is exposed to a teratogen at this critical point, then a congenital anomaly called a tracheal-esophageal (TE) fistula could result. In this anomaly, the esophagus is attached to the trachea rather than to the stomach.^[11] Table 8.4 summarizes the weekly changes in embryo development.

Table 8.4 Embryonic Stage of Development^[12]

Fetal Period

When the embryo reaches nine weeks gestation, it is called a fetus until birth. At the beginning of this stage, the fetus is about the size of a kidney bean and begins to take on the recognizable form of a human being. See Figure 8.16^[13] for an image of an early fetus.

Mothers experience quickening, the first feeling of movement of the fetus in utero, around 16 to 20 weeks. By the time the fetus reaches the sixth month of development (23-24 weeks), it weighs up to 1.4 pounds. Hearing has developed, so the fetus can respond to sounds. The internal organs, such as the lungs, heart, stomach, and intestines, have formed enough that a fetus born prematurely at this point has a chance of survival with intensive medical care.

Between the seventh and ninth months, the fetus is primarily preparing for birth. It is exercising its muscles, and its lungs begin to expand and contract. Around 36 weeks, the fetus is almost ready for birth. It weighs about six pounds and is about 18.5 inches long. By Week 37 all organ systems are developed enough that the fetus could survive outside the mother’s uterus without many of the risks associated with premature birth. The fetus continues to gain weight and grow in length until approximately 40 weeks. See Figure 8.17^[14] for an illustration of embryonic and fetal development. This illustration shows the development of the organs, structures, and tissues of the embryo and fetus, as well as when organs or structures are most vulnerable to teratogens.

Pregnancy

A full-term pregnancy lasts approximately 270 days (approximately 38.5 weeks) from conception to birth. Because it is easier to remember the first day of the last menstrual period (LMP) than to estimate the date of conception, obstetricians set the due date as 284 days (approximately 40.5 weeks) from the LMP. This assumes that conception occurred on Day 14 of the woman’s cycle. These 40 weeks of pregnancy are usually discussed in terms of three trimesters, each trimester lasting approximately 13 weeks. Providing nursing care for pregnant women is further discussed in the “Antepartum” chapter. See Figure 8.18^[16] for an image of a pregnant woman.